// Cache the div so that the browser doesn't have to find it every time the window is resized.
var $div = $('li.leaf.item');
// Run the following when the window is resized, and also trigger it once to begin with.
$(window).resize(function () {
// Get the current height of the div and save it as a variable.
var height = $div.height();
// Set the font-size and line-height of the text within the div according to the current height.
$div.css({
'font-size': (height/2) + 'px',
'line-height': height + 'px'
})
}).trigger('resize');​